I have been doing some reading on different weight distribution systems as I am preparing to buy a new hitch. I have decided on a Reese dual cam setup. Truck is a commercial cab and chassis 3500 flatbed that weighs in at 9500 lbs unloaded.

For the past 6 years (150,000 miles+), I have been towing our 30 bunkhouse on a simple 2 5/16 heavy duty hitch ball, without any sway control or weight distribution. We have never had a sway or weight issue, we switch between several heavy trailers on a regular basis using the same hitch...

We recently bought another truck and it, along with the hitch, has been shipped to the Caribbean with our boat (where we will be working for a few years). The flatbed is still in the states with the Airstream, so I figured it is time for a weight distribution hitch.

My question is due to the size of our tow vehicle, should I go with the lightest bars I can find (600 lbs)? Any input would be appreciated.

Sounds like all you need is the sway part. The weight distribution part for what you have would not be needed. So yes, I would use the 600 lbs. bars.
You might want to look into the airbag hitch set up??